Holographic memory using beam steering

ABSTRACT

A method, apparatus, and system provide the ability for storing holograms at high speed. A single laser diode emits a collimated laser beam to both write to and read from a photorefractice crystal. One or more liquid crystal beam steering spatial light modulators (BSSLMs) steer a reference beam, split from the collimated laser beam, at high speed to the photorefractive crystal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 120 of thefollowing co-pending and commonly-assigned U.S. utility patentapplication, which is incorporated by reference herein:

Utility application Ser. No. 10/824,722, filed Apr. 15, 2004, byTien-Hsin Chao, Jay C. Hanan, George F. Reyes, and Hanying Zhou,entitled HOLOGRAPHIC MEMORY USING BEAM STEERING, attorneys' docketnumber 176.18-US-U1/CIT-3875, which application claims the benefit under35 U.S.C. Section 119(e) of the following co-pending andcommonly-assigned U.S. provisional patent application(s), which is/areincorporated by reference herein:

Provisional Application Ser. No. 60/463,821, filed on Apr. 18, 2003, byTien-Hsin Chao, Hanying Zhou, and George F. Reyes, entitled “COMPACTHOLOGRAPHIC DATA STORAGE SYSTEM,” attorneys' docket number 176.18-US-P1(CIT-3875-P); and

Provisional Application Ser. No. 60/535,205, filed on Jan. 9, 2004, byTien-Hsin Chao, Jay C. Hanan, and George F. Reyes, entitled “HIGHDENSITY HIGH RATE HOLOGRAPHIC MEMORY USING A MEMS MIRROR BEAM STEERINGDEVICE,” attorneys' docket number 176.18-US-P2 (CIT-3875-P2).

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

The invention was made with Government support under Grant No. NAS7-1407awarded by NASA. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to holography, and inparticular, to a holographic memory system using a mirror beam steeringdevice

2. Description of the Related Art

Many devices (e.g., compact discs and digital video discs) use light tostore and read data. However, prior art optical storage methods havelimited transfer and capacity capabilities. To overcome thedisadvantages of the prior art, holographic memory may be used.Holographic memory stores information beneath the surface of therecording medium and uses the volume of the recording medium forstorage. However, holographic memory may also have speed limitationswith respect to recording data and/or reading the data from the storagemedium. These problems may be better understood by describing the futureneeds for memory and prior art holographic memory systems.

Current technology, as driven by the personal computer and commercialelectronics market, is focusing on the development of variousincarnations of Static Random Access Memory (SRAM), Dynamic RandomAccess Memory (DRAM), and Flash memories. Both DRAM and SRAM arevolatile. Their densities are approaching 256 Mbits per die. Advanced3-D multichip module (MCM) packaging technology has been used to developsolid-state recorder (SSR) with storage capacity of up to 100 Gbs. Theflash memory, being non-volatile, is rapidly gaining popularity.Densities of flash memory of 256 Mbits per die exist in the prior art.High density SSR could also be developed using the 3-D MCM technology.However, flash memory is presently faced with two insurmountablelimitations: limited endurance (breakdown after repeated read/writecycles), and poor radiation-resistance (due to simplification in powercircuitry for ultra-high density package).

NASA's future missions may require massive high-speed onboard datastorage capability to support Earth Science missions. With regard toEarth science observation, a 1999 joint Jet Propulsion Laboratory andGoddard Space Flight Center (GFSC) study (“The High Data Rate InstrumentStudy”) has pointed out that the onboard science data (collected by highdate rate instruments such as hyperspectral and synthetic apertureradar) stored between downlinks would be up to 40 terabits (Tb) by 2003.However, onboard storage capability in 2003 is estimated at only 4 Tbthat is only 10% of the requirement. By 2006, the storage capability islikely to fall further behind and supporting merely 1% of the onboardstorage requirements.

Accordingly, prior art electronic memory cannot satisfy all NASA missionneeds. Thus, what is needed is a new memory technology that wouldsimultaneously satisfy non-volatility, rad-hard, long endurance as wellas high density, high transfer rate, low power, mass and volume to meetall NASA mission needs.

Volume holography has been predominantly considered as a high-densitydata storage technology. With volume holography, the volume of therecording medium is utilized for storage instead of only utilizing thesurface area (such as with compact discs [CDs] and/or digital videodiscs [DVDs]). Traditionally, when a laser is fired, a beam splitter isutilized to create two beams. One beam, referred to as the object orsignal beam/wavefront travels through a spatial light modulator (SLM)that shows pages of raw binary data as clear and dark boxes. Theinformation from the page of binary code is carried by the signal beamto a light-sensitive lithium-niobate crystal (or any other holographicmaterials such as a photopolymer in place of the crystal). The secondbeam (produced by the beam splitter), called the reference beam,proceeds through a separate path to the crystal. When the two beamsmeet, the interference pattern that is created stores the data carriedby the signal beam in a specific area in the crystal as a hologram (alsoreferred to as a holographic grating).

Depending on the angle of the reference beam used to store the data,various pages of data may be stored in the same area of the crystal. Toretrieve data stored in the crystal, the reference beam is projectedinto the crystal at exactly the same angle at which it entered to storethat page of data. If the reference beam is not projected at exactly thesame angle, the page retrieval may fail. The beam is diffracted by thecrystal thereby allowing the recreation of the page that was stored atthe particular location. The recreated page may then be projected onto acharge-coupled device (e.g., CCD camera), that may interpret and forwardthe data to a computer.

Thus, as described above, a complex data-encoded signal wavefront isrecorded inside a media as sophisticated holographic gratings byinterference with a selective coherent reference beam. The signalwavefront is recovered later by reading out with the same correspondingreference beam.

Bragg's law determines that the diffracted light intensity issignificant only when the diffracted light is spatially coherent andconstructively in phase. Bragg's law is often used to explain theinterference pattern of beams scattered by crystals. Due to the highlyspatial and wavelength Bragg selectivity of a crystal, a large number ofholograms can be stored and read out selectively in the same volume.Accordingly, there is a potential for one bit per wavelength cube datastorage volume density and intrinsic parallelism of data accessing up toMbytes per hologram.

Accordingly, as described above, the prior art fails to providesufficient memory capabilities. Prior art holographic memory systemshave evolved in an attempt to provide such capabilities. However, theprior art holographic memory systems may still be improved in storagecapacity, efficiency, speed, resistance to radiation, etc.

SUMMARY OF THE INVENTION

An advanced holographic memory technology enables high-density andhigh-speed holographic data storage with random access during datarecording and readout. Embodiments of the invention provide twoelectro-optic beam steering schemes: one utilizing a liquid crystal (LC)beam steering device and the other utilizing a MEMS mirror scanner(Micro-Electro-Mechanical Systems).

Embodiments of the invention may utilize two LC beam steering spatiallight modulators cascaded in an orthogonal configuration to form a twodimensional angular-fractal multiplexing scheme. Alternatively, the MEMSmirror may scan a reference beam (split from a single collimated laserbeam) along a horizontal plane in parallel with a C-axis. Further, theMEMS mirror may be varied by small increments with respect to each newdata page to specifically orient the reference beam to thephotorefractive crystal (which is used to store the holograms) in anangular multiplexing scheme.

In addition, the system may be implemented in a CD-size holographicmemory breadboard. An architecture of the invention may also provide forusing a single collimated laser beam to both write to and read from thestorage device (e.g., the photorefractive crystal). Such a single laserbeam configuration is distinguishable from the prior art configurationswhich normally require multiple different laser diodes/sources. Further,embodiments may also utilize a key Fe:LiNbO₃ photorefractive crystal asthe storage means. Such a storage means has shown significant radiationresistance performance. One or more embodiments of the invention mayalso be used/configured for use with both analog and digital holograms.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 illustrates a schematic architecture that utilizes a liquidcrystal BSSLM in accordance with one or more embodiments of theinvention;

FIG. 2 illustrates electro-optic beam steering in accordance with one ormore embodiments of the invention;

FIG. 3 illustrates beam steering using a phase modulation SLM with avariable grating period in accordance with one or more embodiments ofthe invention;

FIG. 4 is a photograph of an example liquid crystal BSSLM and amagnified view of the grating structure of the SLM in accordance withone or more embodiments of the invention;

FIG. 5A shows an example of a driving voltage waveform profile that maybe used to achieve a very high diffraction efficiency (>80%) for asteered beam in accordance with one or more embodiments of theinvention;

FIG. 5B illustrates an example of a beam steering trace recorded using aBSSLM in accordance with one or more embodiments of the invention;

FIG. 6 illustrates a system architecture of an optical correlator usingholographically stored and retrieved filter data for real-time opticalpattern recognition in accordance with one or more embodiments of theinvention;

FIG. 7A illustrates a set of training images selected for developingMACH correlation filters in accordance with one or more embodiments ofthe invention;

FIG. 7B illustrates the image of one of the developed MACH filters (with8-bit dynamic range) in accordance with one or more embodiments of theinvention;

FIG. 8 illustrates experimental results of pattern recognition of a testflight vehicle obtained using a holographically stored MACH filter inaccordance with one or more embodiments of the invention;

FIG. 9A is a photograph of a book-sized 1-D holographic memorybreadboard in accordance with one or more embodiments of the invention;

FIG. 9B-9D are photographs of a CD-sized compact holographic memorybreadboard with 2D electro-optical angular-fractal beam steering asillustrated in accordance with one or more embodiments of the invention;

FIG. 9E is a photograph that illustrates the use of the grayscaleToutatis Asteroid image sequence for benchmark testing in accordancewith one or more embodiments of the invention;

FIGS. 10A-10C illustrate a canidate MEMS mirror, the packaged system,and its corresponding driving voltage respectively in accordance withone or more embodiments of the invention;

FIG. 11 illustrates a holographic memory system architecture utilizingthe MEMS mirror for beam steering in accordance with one or moreembodiments of the invention;

FIG. 12 is a radiation hologram alteration parameter plotted using anintegrated density approach for each irradiated hologram in accordancewith one or more embodiments of the invention; and

FIG. 13 is a flow chart that illustrates a method for storing data inholographic memory in accordance with one or more embodiments of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings which form a part hereof, and which is shown, by way ofillustration, several embodiments of the present invention. It isunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the present invention.

Holographic Data Storage

As described above, holographic data storage may store data in a largenumber of holograms inside of a photorefractive crystal. Holograms maybe formed by recording (in a cubic photorefractive crystal) the lightinterference pattern caused by a data beam carrying page data (image orbinary bits) and a reference laser beam. Since these images are storedin the Fourier domain and recorded in three dimensions, massiveredundancy is built into the holograms such that the stored hologramswould not suffer from imperfections in the media or point defects.

The LiNbO₃ photorefractive crystal has been the most mature recordingmaterial for holographic memory due to its uniformity, highelectro-optical coefficient, high photon sensitivity, and commercialavailability. One unique advantage for using holographic data storage isits rad hard (radiation hardened) capability. Holograms stored inphotorefractive crystal have been experimentally proven to be radiationresistant. For example, when a Lithium Niobate holographic memory wasflown in space, the retrieved crystals only suffered surface damage andstill retained their photosensitivity for hologram recordings.

Compact Holographic Memory Using Beam Steering

The key to achieve high-speed data transfer rates in a holographicmemory system is the laser beam steering methodology. Variousmethods/systems may be used to improve the speed using beam steering.

Liquid Crystal Beam Steering Devices

In accordance with one or more embodiments of the invention, a liquidcrystal beam steering spatial light modulator (BSSLM) is used forhigh-speed beam steering. FIG. 1 illustrates a schematic architecturethat utilizes a liquid crystal BSSLM in accordance with one or moreembodiments of the invention. The architecture 100 consists of a writingmodule 102 for multiple hologram recordings and a readout module 104 forhologram readout.

The writing module 102 includes a laser diode 106A as the coherent lightsource, a pair of cascaded BSSLMs 108, one transmissive 108A and onereflective 108B in each pair, for angular multiplexed beam steering, adata SLM 110 for data input for storage, two cubic beam splitters 107Aand 107B for beam forming, and a photorefractive crystal 112 forhologram recording.

The readout module 104 also shares the photorefractive crystal 112. Thereadout module includes a laser diode 106B with the same wavelength asthe writing laser diode 106A, a pair of cascaded BSSLMs 113A and 113B togenerate phase conjugated readout beams (i.e., the readout beam isdirected opposite to the writing beam), the shared photorefractivecrystal 112, a cubic beam splitter, and a photodetector array 114 forrecording the readout holograms. The system uses an angular multiplexingscheme to store multiple holograms and phase-conjugated beams to readout each hologram.

In hologram writing, the collimated laser beam 106A splits into twoparts at the first cubic beam splitter 107A. The horizontally deflectedlight travels across the second cubic beam splitter 107B to read out theinput data after impinging upon the data SLM 110. The data carrying beam109 is then reflected into the PR crystal 112 as the data writing beam.

The remaining part of the laser beam 111 travels vertically, passing aBSSLM 108A and is then reflected to the second reflective BSSLM 108B.Both BSSLMs 108 are 1-dimensional blazed phase gratings capable of beamsteering with an angular deflection determined by the grating periods.By cascading two BSSLMs 108 in orthogonal, 2-dimensional beam steeringcan be achieved. Alternatively, a single 2-D beam steering SLM could beused. The deflected laser beam 111 is directed towards the PR crystal112 to form an interference grating (hologram). Each individual hologramis written with a unique reference angle and can only be read out atthis angle (or its conjugated one). By varying the reference beam angle111 in sequential recording, a very large number of holograms can berecorded in the recording medium.

For hologram readout, an innovative phase conjugation architecture isillustrated in FIG. 1. The phase conjugation scheme enables lenslesshologram readout with minimal distortion (low bit error rate). As shownin FIG. 1, a second pair of transmissive 113A and reflective 113B BSSLMsare used to provide a phase-conjugated readout beam (with respect to thewriting reference beam). After the beam impinges upon the PR crystal112, the diffracted beam from the recorded hologram exits the PR crystal112 back tracking the input data beam path, due to the phase-conjugationproperty. The beam then directly impinges upon the photodetector array114 without the need for focusing optics and reconstructing thecorresponding data page, as was recorded and stored in the PR crystal112.

Electro-Optic Beam Steering

In an alternative embodiment of the invention, electro-optic beamsteering as illustrated in FIG. 2 may be used. Collimated laser beam 202first enters the polarizing beam splitter 204A where it is split intotwo beams. The input beam subsequently passes through the data SLM 206,lens 208A, mirror 210A, mirror 210B, mirror 210C, lens 208B, and thenenters the PRC 214 (a Fe:LiNbO₃ photorefractive crystal).

The lens pair 208A and 208B will relay the data SLM 206 throughput imageonto the PRC 214. The mirror set 210A-210C fold and increase the lightpath length to make it equal to that of the reference beam.

The reference beam, after exiting the beam splitter 204A, passes throughbeam splitter 204B, BSSLM 212A, beam splitter 204B (again), lens 208C,beam splitter 204C, BSSLM 212B, beam splitter 204C (again), lens 208D,and arrives at PRC 214.

The data beam and reference beam intersect within the volume of the PRC214 forming a 90° recording geometry. Both beams are polarized in thedirection perpendicular to the incident plane (the plane formed by thereference and signal beams). Lens pair 208C and 208D relay the BSSLM212A onto the PRC 214 surface. BSSLM 212A scans the reference beam alongthe horizontal plane (or the x-axis) in parallel with the C-axis. BSSLM212B steers the reference beam in the vertical plane (y-axis, or thefractal plane). During holographic data recording, the interferencepattern formed by each page of input data is recorded in the PR crystal214. The reference beam angle (and location) is altered with eachsubsequent page of input data. During readout, the data beam is shutdown and the reference beam is activated to illuminate the PR crystal214.

Due to the principle of holographic wavefront reconstruction, the storedpage data, corresponding to the specific reference beam angle, may beread out. The readout data beam exits the PRC 214 and passes throughmirror 210D and lens 208E before reaching the photodetector (PD) array216. Note that the lens set 208A, 208B and 208E relays the input SLM 206to the PD array 216. The magnification factor, caused by the lens set,is determined by the aspect ratio between the data SLM 206 and the PDarray 216.

As depicted in FIG. 2, by using two 1-dimensional BSSLMs 212A and 212Bcascaded in an orthogonal configuration, a 2-dimensional angular fractalmultiplexing scheme is formed, in a breadboard setup that enableshigh-density recording and retrieval of holographic data.

In experiments, holograms were first multiplexed with x-direction(in-plane) angle changes while y-direction angle hold unchanged. Afterfinishing the recording of a row of holograms, the y-direction waschanged (perpendicular to the incident plane) angle, and the next row ofholograms was recorded with x-direction angle changes. Both x and yangle changes are fully computer controlled and can be randomlyaccessed. Accordingly, the recording and retrieval of long video clipsof high quality holograms may be conducted.

Advantages of the use of an electro-optic beam steering scheme mayinclude the absence of mechanical motion, high-transfer rate (1 Gb/sec),random access data addressing, low-volume, and low power.

Beam Steering Spatial Light Modulator

The BSSLMs described above may be implemented in a device built upon aVLSI back plane in a ceramic PGA (pin grid array) carrier. A1-dimensional array of 4096 pixels, filled with nematic twist liquidcrystal (NTLC), is developed on the SLM (spatial light modulator)surface. The device aperture is of the size of 7.4 μm×7.4 μm, each pixelis of 1.18 μm×7.4 μm in dimension. The response time of such anembodiment may reach 200 frames/sect.

Further, the NTLC in the above embodiments may be replaced withFerroelectric Liquid Crystal (FLC). The use of FLC may increase thespeed by one order of magnitude (i.e., >2000 frames/sec).

The principle of operation of such a BSSLM is illustrated in FIG. 3.FIG. 3 illustrates beam steering using a phase modulation SLM with avariable grating period. Since the SLM is a phase-modulation device, byapplying proper addressing signals, the optical phase profile 302 (i.e.,a quantized multiple-level phase grating) would repeat over a 0-to-2πramp with a period d. The deflection angle θ of the reflected beam isinversely proportional to d:θ=sin⁻¹(λ/d)where λ is the wavelength of the laser beam. Thus, beam steering can beachieved by varying the period of the phase grating.

For example, if each period d consists of 8 phase steps each with 1.8 μmpixel pitch. The period d will be 14.4 μm. With the operating wavelengthat 0.5 μm, the total beam steering angle will be about ±3.2°. The totalangle of diffraction will be 6.4°. In the next development step, thepixel pitch can be reduced by 0.5 μm and the corresponding total beamsteering angle will be increased to 22.5°.

The diffraction efficiency, η, of this device is:$\eta = \left( \frac{\sin\left( {\pi/n} \right)}{\pi/n} \right)^{2}$Where n: number of steps in the phase profile. For example η˜81% forn=4, and η˜95% for n=8.

The number of resolvable angles of the steered beam can be defined by:M=2m/n+1Where m is the pixel number in a subarray, and n is the minimum numberof phase steps used. For example, the number resolvable angle M of a4096 array (i.e. m=4096) with of 8 phase levels (i.e. n=8) would be 910.One such device may be configured into eight 1×512 subarray due to theresolution limits of the foundry process. Therefore there may only be129 resolvable angles are available for a BSSLM. A photo of an exampleliquid crystal BSSLM and a magnified view of the grating structure ofthe SLM is shown in FIG. 4.

As described above, some advantages of using such a electro-optic beamsteering device for angular multiplexing for holographic data storageinclude, no mechanical moving parts, randomly accessible beam steering,low voltage/power consumption, large aperture operation, and no need forbulky frequency-compensation optics as in AO based devices.

In addition to the above, a custom phase-array profile driver may beused with a LabView™ based system HW/SW controller for the downloadingof a driving profile to the BSSLM. FIG. 5A shows an example of a drivingvoltage waveform profile that may be used to achieve a very highdiffraction efficiency (>80%) for the steered beam. A sample of beamsteering trace recorded using the BSSLM is shown in FIG. 5B.

Holographic Memory Storage Capacity and Transfer Rate

Various different sizes and types of devices may be used in accordancewith embodiments of the invention.

For example, it has been demonstrated that up to 160,000 pages (i.e. 160Gbs of memory) of hologram can be stored in a LiNbO₃ PR crystal with 1cm³ volume using a scanning mirror to create angular multiplexing foreach reference beam. However, the scanning mirror scheme that requiresmechanically controlled moving parts is not suitable for space flight.Accordingly, one or more embodiments of the invention may provide an allelectro-optic controlled angular multiplexing scheme with high-speed andhigh resolution. In this regard, as described above, the invention mayutilize an all-phase beam steering device, the BSSLM.

Both transmissive and reflective BSSLMs may be used in an advancedholographic memory (AHM) system. An example of a transimissive BSSLMdevice is a 1×1024 array with resolvable spots about 64. An example of areflective BSSLM device is a silicon-based 1-D diffractive beam steeringdevice. Such a reflective BSSLM device may be a 1×4096 array, that hasapproximately 128 resolvable spots. Devices with a higher number ofresolvable spots (around 180) may also be provided in accordance withembodiments of the invention. Thus, total resolvable spots from cascadedBSSLMs may be around 11,520. By using two cascaded BSSLMs for beamsteering, a total of more than 10,000 pages of hologram can be storedand readout in a single cubic centimeter of PR crystal. Since each pagecan store about 1000×1000 pixels of data (1 Mbytes), the total storagecapacity can reach 10 Gigabytes.

In another example, a 1×4096 array may be used with an aperture size of7.4 mm×7.4 mm. Alternatively, the array size may be expanded to 2.5mm×2.5 mm (1 in²) and the corresponding array density would be 1×12000.Thus, the number of resolvable angles would be increased to 2666.

From the above information, it may be seen that the Liquid Crystal BSSLMutilized in a holographic memory setup of the invention may beappropriate for high-density holographic storage. With additionalupgrades in BSSLM performance, the total number of the holograms thatcan be recorded in a holographic memory breadboard may easily exceed20,000. Such a holographic breadboard may be configured by recording2000 holograms in each x-dimension row (i.e. the angular direction) and10 rows in y-dimension (i.e. the fractal direction).

The storage capacity of such a holographic memory system, with using theupgraded electro-optic BSSLM, would then exceed 20 Gb for a 1000pixel×1000-pixel input page. It would further increase to 500 Gb byusing a 5000 pixel×5000 pixel input page. Further miniaturization wouldmake enable the reduction of the holographic memory into a 5 cm×5 cm×1cm cube. By stacking a multiple of such holographic memory cubes on amemory card (e.g. 10×10 cubes on each card), a storage capacity of 2-50Tb per card may be achieved. The transfer rate of such a holographicmemory system may range from 200 Mb/sec (200 pages/sec, with a 1 M pixelpage) to 5 Gb/sec (200 pages/sec, with a 25 M pixel page).

Applying Advanced Holographic Memory (AHM) Technology to Support MassiveStorage Needs of Optical Patterns

The AHM technology may support the massive data storage needs of anoptical pattern recognition system. In this regard, gray scale opticalcorrelators have been extensively developed and applied for patternrecognition. The invention provides a compact grayscale opticalcorrelator (GOC) 602 for real-time automatic target recognition (ATR).As shown in FIG. 6, such an optical correlator 602 may employ a LiquidCrystal Spatial Light Modulator (LC SLM) 604, with 8-bit grayscaleresolution for input incoherent-to-coherent image conversion. FIG. 6illustrates a system architecture of an optical correlator 602 usingholographically stored and retrieved filter data for real-time opticalpattern recognition. The readout data containing grayscale MACH (maximumaverage correlation height) filter data from a high-density holographicmemory 606 is directly fed into the filter SLM driver 608 of a GOC 602to enable real-time ATR.

In the Fourier transform plane, a bipolar-amplitude (i.e. real-valued)SLM may be used to encode the correlation filter. The real-valuedcorrelation filter encoding capability has enabled the use of a verypowerful optimum filter computation algorithm, Maximum AverageCorrelation Height (MACH), for distortion invariant correlationcomputation.

One of the major limitations for more versatile ATR using this GOC 602is the severe limitation size limitation of electronic memory. Such aGOC 602 is capable for updating the correlation filter at a rate of 1000frames/sec. Each filter consists of 512-pixel×512-pixel with 8-bitgrayscale resolution. Thus, to operate the correlator 602 at full speed,the filter data throughput will be at 2 Gigabit/sec. This transfer rateis far beyond that of magnetic hard disk. Only SDRAM could be used withadequate data transfer rate. However, to save a modest number of 1000filters on-board, it would need two Gigabits of SDRAM memory. The memoryboard size and power consumption is too excessive for many air andspace-borne systems to accommodate. Therefore, the invention utilizesholographic memory 606 as an alternative memory solution for real-timepattern recognition using a GOC 602.

Unique advantages of using holographic memory system for updatableoptical correlator applications including high storage density, randomaccess, high data transfer rate, and grayscale image storage capability.All these three characteristics very well meet the memory requirementsof a GOC 602.

Experimental Demonstration of Optical Pattern Recognition Using OpticalCorrelator With Holographic Memory

As described above, one or more embodiments of the invention utilize aportable GOC with optically implemented MACH (maximum averagecorrelation height) correlation filters.

An experimental demonstration has illustrated real-time optical patternrecognition. During such an experimental test, a camcorder-sized GOC maybe used to perform real-time pattern recognition. A CHDS (compactholographic data storage) breadboard may be used to store and readoutMACH correlation filters. The experimental steps may be described asfollows. First, a set of training images, as shown in FIG. 7A, may beselected for developing MACH correlation filters. The image of one ofthese MACH filters (with 8-bit dynamic range) to be stored and retrievedfrom a holographic memory system is shown in FIG. 7B. Second, these MACHfilters may be recorded into a CHDS breadboard and subsequently readoutand downloaded into a filter driver of the GOC. The dynamic range of theretrieved holographic filter image may then be carefully preserved toretain the 8-bit resolution.

For real-time optical pattern recognition operation, a large bank ofMACH correlation filter data would be first stored in an acousto-basedholographic memory 606 as shown in FIG. 6. The readout holographic datawould then be directly fed into the filter SLM driver 608 of the GOC 602to support the high-speed filter updating needs.

After the holographically retrieved MACH filter image is downloaded intothe filter SLM 608 of the GOC 602, a video of input scene recorded froma previous flight test, may be fed into the input SLM 604. Sharpcorrelation peaks associated with the input target in various rotations,scale and perspective may be successfully obtained from the correlationoutput. Some of the correlation output results are displayed in FIG. 8.

Holographic Memory Breadboard with 1D and 2D Electro-Optic Beam Steering

One or more embodiments of the invention may be implemented in abook-sized 1-D holographic memory breadboard as illustrated in FIG. 9A.Such an implementation may demonstrate the feasibility of using a BSSLMdevice for beam steering to meet the multiplexing needs duringholographic data recording and retrieval. Further, such a system mayutilize a single BSSLM and can demonstrate 1-D beam steering for angularmultiplexing. In addition to the above, a typical such system maymeasure 30 cm×20 cm×5 cm, the size of a phone book.

Alternatively, embodiments may be implemented in a CD-sized compactholographic memory breadboard with 2D electro-optical angular-fractalbeam steering as illustrated in FIGS. 9B-9D. Such a CD-sized holographicmemory breadboard is a very compact holographic memory module, measuring10 cm×10 cm×1 cm. The compact size of the VLSI based BSSLM together withadvanced optics design enables a drastic reduction in the system volumefrom book-size to CD-size. Such a breadboard is capable of recording 10GB of holographic data. Further, the system design makes it possible foreasy replacement of key devices when an upgraded version becomesavailable. Such key devices include the Spatial Light Modulator, theBSSLM, and the PD (photodetector) array. Moreover, the system storagecapacity may increase by up to 2 orders of magnitude with the use of ahigh-resolution BSSLM.

The CD-sized holographic memory breadboard may be developed with acomprehensive LabView™ based system controller. Hence, autonomous datarecording and retrieval is available upon full integration of thesystem.

FIG. 9E illustrates the use of the grayscale Toutatis Asteroid imagesequence for benchmark testing (i.e., during data storage test andevaluation). Some examples of the retrieved holographic images of theToutatis asteroid, excerpted from a long recorded video clip, are shownin FIG. 9E.

Thus, as described above, an advanced holographic memory technology maybe used to enable high-density and high-speed holographic data storagewith random access during data recording and readout. An innovative E-O(electro-optical) beam steering scheme, achieved by utilizing a liquidcrystal beam steering device has been shown. Further, a CD-sizedholographic memory breadboard may be integrated and used for successfulholographic data recording and retrieval. Such a breadboard is compactwith a storage capacity range from 10 Gb to 250 Gb, depending on theinput page size.

MEMS Mirror for High-Speed Beam Steering

Although the liquid crystal (LC) BSSLM phase array has been successfullyutilized for high-speed beam steering in a compact holographic memorybreadboard, it would be beneficial to improve the light throughputefficiency. Due to the light diffraction of the throughput light beamsby the phase array in a LC BSSLM, there are many diffracted orders(other than the first order of diffracted laser beams) that are used forhologram recording. Since it is very difficult to achieve 100%diffraction efficiency in the first order, a considerable amount oflaser beam energy is spread into the zero order and high order ofdiffraction. The high-order-light beams cause spurious interference thatoften reduces the signal-to-noise ratio of the recorded holograms.

Therefore, one or more embodiments of the invention provides for ahigh-speed scanning mirror that utilizes light deflection instead ofdiffraction as the beam steering device. The prior art illustrates theuse of galvanometer controlled mirrors for laser beam steeringapplications. However, the considerable mass of the galvanometer mirrormay severely limit its scanning speed (e.g. no more than video rate). Inview of the limitations of the prior art, the invention provides for theuse of emerging MEMS (Microelectromechanical Systems) mirror technologyfor high-speed beam steering in a compact holographic memory system.

Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanicalelements, sensors, actuators, and electronics on a common siliconsubstrate through microfabrication technology. While the electronics arefabricated using integrated circuit (IC) process sequences (e.g., CMOS,Bipolar, or BICMOS processes), the micromechanical components arefabricated using compatible “micromachining” processes that selectivelyetch away parts of the silicon wafer or add new structural layers toform the mechanical and electromechanical devices.

MEMS Micro-mirrors are mirrors that have been “shrunk” down to themicroscopic world. Such MEMS Micro-mirrors may be used in many waysincluding application in the field of fiber optics. Alternatively, theMEMS Micro-mirrors may be utilized for beam steering in a holographicmemory system.

The fabrication method for these micro-mirrors is similar (or identical)to that of a cantilever structure except that after the process iscompleted, a reflective layer, such as aluminum, may be placed on top ofthe beam.

A MEMS micro-mirror utilizes electrostatic actuation for mirrorsteering. Since positive and negative charges attract each other (andlike charges repel), if a cantilever can be made to keep a positivecharge while placing an alternating positive-negative charge above it,then by electrostatics, the cantilever will resonate up and down.

In view of the above, a MEMS mirror can be attractive as a beam steeringdevice in a holographic memory system. Advantages of using a MEMS mirroras a beam steering device include: high light throughput efficiency(>99% reflectivity), superior beam quality (light reflected from amirror does not generate spurious diffraction as that of a diffractivebeam steering device), low mass and high-speed.

FIGS. 10A-10C illustrate a candidate MEMS mirror, the packaged system,and its corresponding driving voltage respectively in accordance withone or more embodiments of the invention.

The holographic memory system architecture utilizing the MEMS mirror forbeam steering is shown in FIG. 11. Details of the system layout providethat a collimated laser beam 1100 first enters a polarizing beamsplitter 1102, and on exit is split into two beams. The input beamsubsequently passes through the data SLM (spatial light modulator) 1104,and image relaying lens pair 1106A-1106B (also referred to as lens L1and L2 having focal distances f1 and f2 respectively), then impinges onan Iron doped Lithium Niobate (Fe:LiNbO₃) photorefractive crystal (PRC)1008. The imaging relay lens pair 1106A-1106B is used to scale theimaging size of the input SLM 1104 to match that of the input pupil ofthe PRC 1108. The imaging relay lens pair 1106A-1106B may also sharplyimage the input SLM 1104 image onto the recording plane of a CCD 1110placed behind the PRC 1108.

The PRC 1108 is the holographic recording device capable of largecapacity, rewriteable, holographic memory recording. The other beam(i.e. the reference beam) will first pass through the imaging relay lenspair 1106C-1106D (also referred to as lens L3 and L4 respectively)before impinging upon the MEMS mirror 1112. The laser beam will then bedeflected by the MEMS mirror 1112 by a pre-determined incremental angle.The deflected reference beam will continue to pass through the thirdimaging relay lens pair 1106E-1106F (also referred to as lens L5 and L6respectively) and reach the PRC 1108. The reference beam and the databeam intersect within the volume of the PRC 1108 forming a 90° recordinggeometry. Focal lengths/distances (e.g., f3+f4) and aperture size of thelens pair 1106C-1106D is selected to compensate the scale differencebetween the input SLM 1104 aperture and that of the MEMS mirror 1112.Similarly, the lens pair 1106E-1106F feature dimensions (e.g., focaldistances f5 and f6) that are selected to match the scale differencebetween the MEMS mirror 1112 and the PRC 1108 entrance pupil.

The MEMS mirror 1112 scans the reference beam along the horizontal plane(or the x-axis) in parallel with the C-axis. During holographic datarecording, the deflected angle from the MEMS mirror 1112 is varied by asmall increment with respect to each new data page. Thus, theinterference pattern formed between each page of input data beam and thespecifically oriented reference beam will be recorded in the PR crystal1108 in an angular multiplexing scheme.

During readout, the data beam will be shut down and the reference beamwill be activated to illuminate the PR crystal 1108. Due to theprinciple of holographic wavefront reconstruction, the stored page data,corresponding to a specific reference beam angle, will be readout. Thereadout data beam will be sharply imaged onto the CCD 1110 recordingplane.

Radiation Resistance of the Fe:LiNbO₃ Photorefractive Crystal

Advanced holographic memories for space applications require not onlyhigh-density and high-speed data storage, but also high radiationresistance. Accordingly, due to the inherent redundant nature andradiation self-shielding effect of volume storage, holographic memoriesmay be required to be radiation resistant.

To ensure radiation resistance, various quantitative experimentalmeasurements of the radiation effect of Co⁶⁰ Gamma Radiation on thestored hologram within Fe:LinbO₃ PR Crystal may be taken. To conduct thetest, a grayscale image may be written into Fe:LiNbO₃ crystal. Duringthe recording, this crystal is placed in a precision holder. The crystalholder ensures that hologram readouts from the crystal, before and afterthe radiation test, are acquired under the same experimental setupparameters. This ensures that any deviation between the two readouthologram images is caused only by the radiation effect.

During gamma irradiation and transportation from one place to another,the crystal may be covered with a thin polyethylene bag to protectagainst small particles from the air that may deposit on the crystal.Quantitative measurements on the hologram as an image may be performedusing specialized software for image analysis. Such a program may allowthe selection of the image and the calculation of the integrated densityof the image throughput intensity, that is the sum of the gray values inthe selection, with background subtracted. Accordingly, the integrateddensity can be computed using the following formula:Integrated Density=N*(Mean−Background)Where N is number of pixels in the selection, and Background is themodal gray value (most common pixel value) after smoothing thehistogram. Using the integrated density approach for each irradiatedhologram, the radiation hologram alteration parameter plotted in FIG. 12may be obtained.

As shown in FIG. 12, holographic memory stored in Fe(0.10%):LiNbO3crystal shows radiation resistance to Co⁶⁰ gamma radiation. Such resultsillustrate that a hologram recorded in a highly Fe doped crystal, about0.10% wt. Fe, is affected very little by radiation with a dose up to 400krad. Further, the maximum change in radiation-altered hologram,2.5×10⁻⁴ is reasonably low. Such a preliminary radiation test shows thatthe Fe:LinbO₃ photorefractive material is at least four times moreradiation resistant than its electronic counterpart.

Logical Flow

FIG. 13 is a flow chart that illustrates a method for storing data inholographic memory. At step 1300, a single laser diode emits acollimated laser beam for both writing a hologram to and reading thehologram from a photorefractive crystal. At step 1302, the collimatedlaser beam is split into a reference beam and an input beam. At step1304, one or more liquid crystal beam steering spatial light modulators(BSSLMs) or Micro-Electro-Mechanical Systems (MEMS) mirrors are used tosteer the reference beam at high speed to the photorefractive crystal.At step 1306, the hologram is stored/recorded in the photorefractivecrystal in a form of an interference pattern created by the steeredreference beam and the input beam.

In accordance with embodiments of the invention, the BSSLMs may comprisetwo BSSLMs cascaded in an orthogonal configuration to form a twodimensional angular-fractal multiplexing scheme. Alternatively, the MEMSmirror may steer the reference beam by scanning the reference beam alonga horizontal plane in parallel with a C-axis. In this regard, duringwriting to the photorefractive crystal, the MEMS mirror may be varied bya small increment with respect to each new data page to specificallyorient the reference beam to the photorefractive crystal in an angularmultiplexing scheme. Further, the components of the system may beimplemented/configured in a CD-sized holographic memory breadboard.Additionally, the data may be stored in the hologram in either analog ordigital form and the photorefractive crystal may comprise Fe:LiNbO₃photorefractive material.

CONCLUSION

This concludes the description of the preferred embodiment of theinvention. In accordance with embodiments of the invention, an advancedholographic memory technology may be used to enable high-density andhigh-speed holographic data storage with random access during datarecording and readout. Two innovative electro-optical beam steeringschemes are described herein: one utilizing a liquid crystal beamsteering device, and the other utilizing a MEMS mirror scanner.

The invention also provides a CD-sized holographic memory breadboardthat may be used for successful holographic data recording andretrieval. In addition, the invention provides an innovative high-speedbeam steering technology using a MEMS mirror. Such a high efficiency,compact MEMS mirror, may further enable the development of an even morecompact and high-density holographic memory system.

The invention also illustrates how testing may be performed on Fe:LinbO₃photorefractive crystal. Gamma radiation tests on a series of the PRcrystal may be conducted with different doping concentrations. Byidentifying the proper doping level the most radiation resistanceperformance may be explored.

In view the above, the use of either a liquid crystal BSSLM or MEMSmirror to steer the reference beam, the invention utilizes a device thatessentially has no moving parts. Such a configuration providessignificantly increases the speed for storing/writing and readingholograms stored in the photorefractive material.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

1. A holographic memory system comprising: (a) a photorefractive crystalconfigured to store holograms; (b) a single laser diode configured toemit a collimated laser beam to both write to and read from thephotorefractice crystal; and (c) one or more liquid crystal beamsteering spatial light modulators (BSSLMs) configured to steer areference beam, split from the collimated laser beam, at high speed tothe photorefractive crystal.
 2. The system of claim 1, wherein the oneor more liquid crystal BSSLMs comprise two BSSLMs cascaded in anorthogonal configuration to form a two dimensional angular-fractalmultiplexing scheme.
 3. The system of claim 1, wherein thephotorefractive crystal, single laser diode, and liquid crystal BSSLMsare implemented in a CD-sized holographic memory breadboard.
 4. Thesystem of claim 1, wherein the reference beam and an input beam,obtained from the collimated laser beam, create an interference patternin the photorefractive crystal to record the hologram.
 5. The system ofclaim 1, wherein the holographic memory system is configured for usewith both analog and digital holograms.
 6. A method for storing data inholographic memory comprising: a single laser diode emitting acollimated laser beam for both writing a hologram to and reading thehologram from a photorefractive crystal; splitting the collimated laserbeam into a reference beam and an input beam; one or more liquid crystalbeam steering spatial light modulators (BSSLMs) steering the referencebeam at high speed to the photorefractive crystal; storing the hologramin the photorefractive crystal in a form of a interference patterncreated by the steered reference beam and the input beam.
 7. The methodof claim 6, wherein the one or more liquid crystal BSSLMs comprise twoBSSLMs cascaded in an orthogonal configuration to form a two dimensionalangular-fractal multiplexing scheme.
 8. The method of claim 6, whereinthe photorefractive crystal, single laser diode, and liquid crystalBSSLMs are implemented in a CD-sized holographic memory breadboard. 9.The method of claim 6, wherein data may be stored in the hologram ineither analog or digital form.
 10. An apparatus for storing data inholographic memory comprising: means for storing a hologram; means foremitting a collimated laser beam to both write to and read from themeans for storing the hologram; and one or more liquid crystal beamsteering spatial light modulators (BSSLMs) configured to steer areference beam, split from the collimated laser beam, at high speed tothe means for storing the hologram.
 11. The apparatus of claim 10,wherein the one or more liquid crystal BSSLMs comprise two BSSLMscascaded in an orthogonal configuration to form a two dimensionalangular-fractal multiplexing scheme.
 12. The apparatus of claim 10,wherein the means for storing the hologram, means for emitting acollimated laser beam, and the one or more liquid crystal BSSLMs areimplemented in a CD-sized holographic memory breadboard.
 13. Theapparatus of claim 10, wherein the reference beam and an input beam,obtained from the collimated laser beam, create an interference patternin the means for storing the hologram to record the hologram.
 14. Theapparatus of claim 10, wherein the apparatus is configured for use withboth analog and digital holograms.